Conventional magnetic recording disks in hard disk drives typically use a continuous granular magnetic film, such as a sputter-deposited hexagonal-close-packed (HCP) cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetic bit in the medium is comprised of many small magnetized grains.
The challenge of producing continuous granular films as magnetic media will grow with the trend toward higher areal storage densities. Reducing the size of the magnetic bits while maintaining a satisfactory signal-to-noise ratio, for example, requires decreasing the size of the grains. Unfortunately, significantly reducing the size of weakly magnetically coupled magnetic grains will make their magnetization unstable at normal operating temperatures. To postpone the arrival of this fundamental "superparamagnetic" limit and to avert other difficulties associated with extending continuous granular media, there has been renewed interest in patterned magnetic media.
With patterned media, the continuous granular magnetic film that covers the disk substrate is replaced by an array of spatially separated discrete magnetic regions or islands, each of which serves as a single magnetic bit. The primary approach for producing patterned media has been to selectively deposit or remove magnetic material from a magnetic layer on the substrate so that magnetic regions are isolated from one another and surrounded by areas of nonmagnetic material. There are a variety of techniques for the selective deposition or removal of magnetic material from a substrate. In one technique the substrate is covered with a lithographically patterned resist material and a magnetic film is deposited to cover both the areas of resist and the areas of exposed substrate. The resist is dissolved to lift off the magnetic film that covers it, leaving an array of isolated magnetic regions. An alternative technique is to first deposit a magnetic film on the substrate and then pattern resist material on the magnetic film itself. Magnetic material from the areas not protected by the resist can then be selectively removed by well-known processes. Examples of patterned magnetic media made with these types of lithographic processes are described in U.S. Pat. Nos. 5,587,223; 5,768,075 and 5,820,769.
From a manufacturing perspective, an undesirable aspect of the process for patterning media that requires the deposition or removal of material is that it requires potentially disruptive processing with the magnetic media in place. Processes required for the effective removal of resists and for the reliable lift-off of fine metal features over large areas can damage the material left behind and therefore lower production yields. Also, these processes must leave a surface that is clean enough so that the magnetic read/write head supported on the air-bearing slider of the disk drive can fly over the disk surface at very low flying heights, typically below 30 nanometers (nm).
A technique for patterning a special type of perpendicular magnetic recording media by ion irradiation is described by Chappert et al, "Planar patterned magnetic media obtained by ion irradiation", Science, Vol. 280, Jun. 19, 1998, pp. 1919-1922. In this technique Pt-Co-Pt multilayer sandwiches which exhibit perpendicular magnetocrystalline anisotropy are irradiated with ions through a lithographically patterned mask. The ions mix the Co and Pt atoms at the layer interfaces and substantially reduce the perpendicular magnetocrystalline anisotropy of the film.
In contrast to conventional CoPt alloy continuous granular films, chemically-ordered alloys of FePt and CoPt formed as thin films have also been proposed for horizontal magnetic recording media. Chemically-ordered alloys of FePt and CoPt, in their bulk form, are known as tetragonal L1.sub.0 -ordered phase materials (also called CuAu materials). They are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are also desirable for high-density magnetic recording media. The c-axis of the L1.sub.0 phase is similar to the c-axis of HCP CoPt alloys in that both are the easy axis of magnetization. Thus, while the disordered face-centered-cubic (FCC) solid solution of Co and Pt has cubic symmetry and low magnetocrystalline anisotropy, the ordered L1.sub.0 phase has uniaxial anisotropy similar to, but greater in magnitude than HCP CoPt alloys.
These chemically-ordered films can be made by several known processes. Films having the L1.sub.0 phase of FePt with the c-axis oriented perpendicular to the substrate, and thus suitable for magneto-optical or perpendicular magnetic recording media, have been grown by molecular beam epitaxy, and by alternating the deposition of layers of Fe and Pt, followed by annealing, the latter approach being described in U.S. Pat. No. 5,363,794. In another approach, equiatomic FePt or CoPt is sputter deposited as a continuous film and then subjected to a relatively high-temperature post-deposition annealing to achieve the chemical ordering. This approach results in the c-axis being oriented substantially in the plane of the film, so that the films are suitable for horizontal magnetic recording, and is described by Coffey et al., "High Anisotropy L1.sub.0 Thin Films for Longitudinal Recording", IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995, pp. 2737-2739; and Watanabe et al., "Microstructure and Magnetic Properties of High-Coercive Fe-Pt Alloy Thin Films", Materials Transactions, JIM, Vol. 37, No. 3, 1996, pp. 489-493. In a third approach, as described in IBM's co-pending application Ser. No. 08/920,938 filed Aug. 29, 1997, now U.S. Pat. No. 6,086,974, which is incorporated herein by reference, a continuous granular film with grains of a chemically-ordered FePt or FePtX (or CoPt or CoPtX) alloy in the tetragonal L1.sub.0 structure and with the c-axis in the plane for horizontal magnetic recording, is produced by sputtering without annealing. The granular structure and chemical ordering are controlled by sputter parameters and by the use of an etched seed layer that provides a structure for the subsequently sputter-deposited granular magnetic film.
What is needed is a horizontal magnetic recording media that achieves the advantages of both patterned media and the chemically-ordered type of media and that can be manufactured without selectively removing and/or depositing the magnetic material on the substrate.